A DVM, or digital video master, usually in the form of digital tape and usually containing an accompanying digital audio track, is commonly the end result of the process of creating material for visual display with electronic devices. The original source of the material might be film, video, computer-generated or any other medium for capturing visual and audio data. Typically, the original material has been processed in a variety of formats during post production, including, but not limited to, ordinary editing, the introduction of fades and other special effects, color correction, and audio adjustments. The DVM is the digital video representation of the visual and audio material in a form suitable for commercial release, or perhaps at some important junction prior to final release.
Generally, more than one copy of the DVM is required. For example, when film material is brought to a post-production house for transfer to video, editing, audio work, etc., the client may eventually wish to have several copies of the DVM. Other situations in which multiple copies are necessary include archiving and sending copies to multiple locations and distributors. It is therefore necessary to create physical copies of the DVM, and it is desirable that each copy, or “dub,” be the same. Indeed, in principle, each copy should be an exact replication of the original DVM.
Due to imperfections in standard, tape-based reproduction processes, it is often the case that some differences exist between each individual copy and the original. The principal reason is that the electronic machines which process digital tape operated at very high speeds and occasionally make faulty recordings causing random errors in the duplicate. When the digital data is derived from a computer based source, pixel noise may occur and cause errors. Some mistakes, e.g., small changes in the color values in isolated locations, are relatively benign in the sense that the copy is visually indistinguishable from the original by a typical viewer. Other alterations are more serious, such as dropping a number of lines or an entire field (each video frame consists of two half-frames called fields, a field consisting of every other horizontal line so that the first field contains the even lines of the frame and the second field contains the odd lines of the frame), or introducing a sustained stretch of visual or audio noise or artifacts.
One current solution is manually intensive quality control. Each copy of the DVM is carefully checked by a trained specialist. The operator watches the video material on a monitor, perhaps at reduced speed, and might step through the material frame-by-frame in suspect sections. The operator then decides whether or not to accept the copy as a faithful reproduction. A similar process may also be performed on an audio track.